The present invention relates to a fluid filter arrangement for a motor vehicle drive train, having a filter housing, which has a fluid inlet and a fluid outlet, allowing fluid to flow in one flow direction from the fluid inlet to the fluid outlet, and having a filter insert, which is arranged in the filter housing and divides an interior of the filter housing into an inlet region connected to the fluid inlet and an outlet region connected to the fluid outlet, wherein the filter insert has a filter medium for filtering fluid.
The present invention furthermore relates to a method for filtering a drive train fluid, in particular by means of a fluid filter arrangement of the type designated above, comprising the step of conveying a fluid through a filter medium, which is arranged in a filter housing.
Transmissions for motor vehicle drive trains generally contain a fluid, such as an oil, which is used for the purpose of lubrication and/or cooling. In addition to bath-type lubrication, in which lubrication and cooling are accomplished by virtue of the fact that rotatable elements of the transmission, such as gearwheels, dip into a fluid sump, injection-type lubrication is also known.
In the widest sense, an injection-type lubrication system contains a fluid supply device, which delivers fluid from the fluid sump and conveys it in a targeted manner to points within the transmission which are to be lubricated and/or cooled. Fluid supply devices of this kind generally have a pump, the suction side of which is connected to the fluid sump.
Another known practice in transmissions for motor vehicle drive trains is to provide hydraulic actuators, which can be used to actuate a clutch arrangement and/or to actuate selector clutches or the like, for example. The hydraulic pressure required for this purpose is produced via a fluid supply device, which is likewise generally equipped with a pump.
Fluid supply devices of the type described above can have pumps which are coupled to an auxiliary drive unit of a drive motor. Here, the speed of the pumps generally corresponds to the speed of the drive motor. However, the practice of driving pumps of this kind independently of the vehicle drive motor, e.g. by means of a respectively dedicated electric motor, by means of which the pump can be driven in one or two directions of rotation, is also known. In the latter case, the pump can also be used as an actuator pump, such that the pressure made available to a hydraulic actuating cylinder or the like on the delivery side of the pump is regulated by means of the speed of the pump. In this case, it is not necessary to arrange a proportional pressure control valve or the like between the pump and the actuating cylinder. This can make the drive train less expensive overall since a hydraulic branch designed in this way does not require any proportional valves, which can only be installed in the assembly environment if cleanliness requirements are increased.
In the case of the abovementioned pumps, a suction side is generally connected to a fluid sump. The fluid sump can be a fluid sump of the transmission but can also be a combined fluid sump of a transmission and of a wet clutch arrangement, as in the case of some dual clutch transmissions for example.
To avoid a situation where the pump draws in contaminants from the fluid sump, the suction side of the pump is generally connected to the fluid sump via a fluid filter arrangement of the type described at the outset, such that a fluid inlet of the fluid filter arrangement is connected to the sump and a fluid outlet of the fluid filter arrangement is connected to a suction side of a pump of a fluid supply device of this kind.
The main flow direction from the fluid inlet to the fluid outlet set up in a filter housing of a fluid filter arrangement of this kind is preferably directed from the bottom up, i.e. against gravity (in the case of use of the motor vehicle drive train and of the fluid filter arrangement as intended). In this case, the fluid can flow vertically upwards or, alternatively, obliquely upwards through the filter medium.
In some operating states of a motor vehicle drive train of this kind, the fluid in the fluid sump may be highly foamed with air or other gases. Particularly when the fluid is conveyed through the filter medium at relatively low flow velocities, there is the possibility that air or another gas will separate out of the fluid underneath the filter medium and agglomerate there to form relatively large bubbles. Low flow velocities can occur particularly when a pump of a fluid supply device is driven purely according to demand, that is to say, for example, by means of an electric motor.
Gas bubbles of this kind (also referred to below as air bubbles) can pass through the filter medium owing to the buoyancy force thereof when certain sizes are reached and can abruptly enter a pump performing a sucking action. This can lead to severe impairment of the hydraulic pressure on the delivery side of a pump, which can have a disadvantageous effect on the controllability of such pressure, especially if the pump is being used as a pump actuator as described above.
Gas bubbles of the type described above can collect underneath the filter medium, especially in those regions through which the flow is generally weaker than in other areas owing to the geometry of the filter housing or the like.
When using a pump as a pump actuator, the volume flow required by the pump during phases in which the pressure is being finely or accurately regulated is relatively low, and therefore it is precisely in such phases that very low flow velocities can occur in the fluid filter arrangement and hence that the risk of air agglomeration is increased. However, it is precisely in these phases that abrupt penetration of an air bubble through the filter medium has particularly disadvantageous effects since the hydraulic pressure on the delivery side of the pump is thereby greatly impaired in the sense of a disturbance variable.
A fluid filter arrangement of the type in question is known from document DE 102 96 703 T5, for example.